Q: Updating an Older Mac

I am currently using:

• An iMac (21.5-inch, late 2009.)
• Processor: 3.06 GHz Intel core two duo
• Memory 4 GB 1067 MHz DDR3
• Running OS X El Capitan version 10.11.6.

  This Mac was left unused for years in one of the offices in my Police Department. Despite being seven years old,  it still outperforms new PCs my coworkers are using. I would like to update the memory on this Mac, but I am unsure of how much it could handle. 

  I am currently assigned as the training coordinator for Police Department of 140 officers.  So keeping track of the training records as well as other documents have been overwhelming at times.  With the assistance of some more or less inexpensive apps (Hazel, DevonThink Pro, etc.) for the first time in nearly three years, I can finally take a vacation with my family and not worry about going home to a nightmare.  As I learn and become more comfortable with the Apple platform, I find myself using more app that causes the computer to perform slowly. 

Is it possible to remove the two, 2 GB memory cards (equaling  4 GB)?  And install four separate 4 GB memory cards (equaling 16 gigs.) Can this older Mac handle that amount of memory?  If so do you have any recommendations where I could purchase it?

I was also advised to look into getting an SSD card,  again I'm unsure what size I should purchase for this machine.  Any and all help is welcomed. 

Thanks